Plasma radiation at \(\omega _{pe}\)  has been previously identified from the plasma waves solely and impulsively driven by attosecond hot electron bunches produced in periodic vacuum heating [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. In the mixed target, however, the positron plays a significant role and drastically changes the plasma wave excitation and the radiation features. The continuous positron acceleration [Eqs. (REF ) and (REF )] forms a dense positron beam which streams into the plasma. This beam drives a perturbation in the background electrons, where the electrons are attracted by the positive driver; large-amplitude plasma oscillations are therefore generated (shown later). Inside the target, where the laser field vanishes, the positron beam starts to stretch under the electrostatic field of the background plasma wave. A significant number of positrons (about one half), are pulled back by this field, and thus carry negative momenta. This reflux of positrons induces strong plasma waves moving backward to the front side of the target. Counterpropagating plasma waves are therefore created producing \(2\omega _{pe}\)  (and even \(3\omega _{pe}\) )-radiation via inverse two-plasmon decay [9]}, [10]}, [11]}, [12]}. This is also qualitatively different from the conventional scenario [11]}, [14]}, [15]}, [16]}, [7]}, [12]}, [8]} of \(2\omega _{pe}\) -radiation where the backward wave is driven by the hot electrons reflected from the rear surface of the target.
